Weight reduction of transportation machines such as an automobile and a railway vehicle is desired in order to curtail exhaust gas by improvement of fuel consumption. Though usage of a thin steel sheet for a member of the transportation machine is effective in reducing weight of the transportation machine, it is desired that the steel sheet itself has high strength in order to secure desired strength while using the thin steel sheet.
For a member of a transportation machine such as a side frame of a truck, a steel sheet in which a scale (black scale) generated during hot rolling remains is sometimes used in view of a cost or the like. However, in a conventional steel sheet in which a scale remains, the scale may exfoliate in finishing such as passing in leveler equipment or working such as bending and pressing carried out by a user. Exfoliation of a scale necessitates care for a roll or a mold to which the scale attaches. Further, when the scale remains after the care, the scale may be pushed into a steel sheet processed thereafter, to generate a depression pattern in the steel sheet. Therefore, excellent scale adhesion is required of a steel sheet in which a scale remains in order to suppress exfoliation of the scale from a base iron.
Though a steel sheet aiming at improvement of scale adhesion is known, a conventional steel sheet cannot achieve both good mechanical property and excellent scale adhesion.